Vacuum switch contact

ABSTRACT

A vacuum switch for making and breaking an inductive electric circuit is composed of a highly evacuated envelope and a pair of separable electrodes located within the envelope. At least one of the contacts of the electrodes is made of a sintered alloy essentially composed of a metal carbide, such as tungsten carbide, having low work function, another electrically conductive metal, such as silver, which also offers good machining characteristics, and certain wettable materials which allow the silver to impregnate the tungsten carbide by filling in the interstices between the particles thereof. When the contacts are opened, or separated, the resultant heat of an arc initiated therebetween causes the tungsten carbide to emit thermionic electrons which are effective for ionizing the gas, or metal vapors, emanating from the contact surfaces, whereupon the arc is maintained until the first current zero is reached even when only small currents are flowing through the same and chopping problems are generally prevented.

United States Patent Nabae et al.

[54] VACUUM SWITCH CONTACT [72] Inventors: Akira NabaefMitsui-u Arii,both of Tokyo; Osamu Arakawa; Sadao Sugiyama, both of Yokohama, all ofJapan [73] Assignee: Tokyo Shibaure Electric Co., Ltd.,

' Kawasaki-shi, Japan [22] Filed: March 27, 1970 [21] Appl. No 23,392

UNITED STATES PATENTS 2,975,256 3/1961 Lee et al. ..200/l44 B 3,014,11012/1961 Cobine ..200/l44 B 3,254,189 5/1966 Evanicsk0,.1r.

et al ..200/l66 C FOREIGN PATENTS OR APPLICATIONS 1,079,013 8/1967 GreatBritain ..200/l44 B 1 Aug. 8, 1972 Primary ExaminerRobert S. MaconAttorney-Oblon, Fisher and Spivak [57] ABSTRACT A vacuum switch formaking and breaking an inductive electric circuit is composed of ahighly evacuated envelope and a pair of separable electrodes locatedwithin the envelope.

At least one of the contacts of the electrodes is made of a sinteredalloy essentially composed of a metal carbide, such as tungsten carbide,having low work function, another electrically conductive metal, such assilver, which also offers good machining characteristics, and certainwettable materials which allow the silver to impregnate the tungstencarbide by filling in the interstices between the particles thereof.when the contacts are opened, or separated, the resultant heat of an arcinitiated therebetween causes the tungsten carbide to emit thermionicelectrons which are effective for ionizing the gas, or metal vapors,emanating from the contact surfaces, whereupon the arc is maintaineduntil the first current zero is reached even when only small currentsare flowing through the same and chopping problems are generallyprevented.

2 Claims, 1 Drawing Figure FATENTED 3m? 3.683.138

WVENTORS AKIRA NABAE MITSURU ARH OSAMU ARAKAWA SADAO SUGIYAMA 06M,FPS/L44 3 ATTQRNEYS 1 VACUUM swmzrr CONTACT BACKGROUND OF THE INVENTIONThe present invention relates to electrical vacuum switches, and moreparticularly to a vacuum circuit interrupter'of the type having a highlyevacuated envelope and an enclosed pair of separable electrodes formaking and breaking an electric circuit connected in series therewithwhich possesses improved. arc maintenance characteristics in typicallylow current interruptions.

As is well known, vacuum type switches are widely used as currentinterrupters in alternating current circuits and in certain oscillationcircuits wherein the oscillatory current form is one which typicallyoscillates between a substantially zero current level and a currentlevel offset from the zero level line.

The electrical vacuum switches referred to herein are of the typenormally employed for switching on and off" operations of an electriccircuitinvolving the inductive loads therein, such as, for example,induction motors, synchronous motors, transformers, induction furnacesand the combination thereof.

Upon the consideration, particularly of the interruption of current insuch inductive alternating current circuits employing vacuum switches ofthe type described having separable main electrodes disposed within anevacuated envelope, it is most important that the actual extinction ofthe electric arcs maintained between the separated contact surfaces ofthe electrodes is carried out at a time at which the load currentflowing through the electrodes is of substantially zero amplitude.Accordingly, under ideal conditions, after separation of the contacts ofan interrupter, an arc should persist until a natural current zero isreached. What has happened in the past, however, is that prematureextinction, or chopping, of the alternating current arc has occurredbecause of vapor starvation near the electrodes, in certain instances.where the load current suddenly falls off or at low current operatinglevels. This means that the rate of change of the load currents in theinductive. circuit is theoretically infinitive, and, when such occurs,the electric installations in the circuit may be damaged by a dangerousvoltage surge thereby created which is capable of easily destroying theinsulations thereof by the product of the higher induced voltages due tothe inductances in the circuit and the extremely large rate of change ofthe currents occurring in the same.

This sudden cutting phenomena of the current flowing through theseparated electrodes is referred to as a so-called chopping of the loadcurrents, as indicated above, and the current level at which thechopping occurs is referred to as the chopping current level.

To prevent the breakdown of the insulations in electric machines andinstruments connected in the circuit with the vacuum switches, a type ofvacuum switch has been provided in which a contact material having ahigh metal vapor pressure is employed in the arc regions of theelectrodes, The representative example of such contact material is analloy consisting of copper as the major constituent and a substantialamount of bismuth as the minor one, extending to about 20 percent byweight of the entire amount of the contact material.

As indicated hereinabove, in circuits employing these vacuum typeswitches, the current chopping phenomena is most likely to occur when aload current having a relatively small amplitude, such as, for example,one of 10 amperes, or less, is flowing through the separated electrodes.This results from the insufficiency of the metal vapors in the vicinityof the separated contact surfaces of the electrodes due to a relativelylower energy of lower current arc.

According to our experiences, although the aforesaid alloy of Cu and Bihas the requisite high metal vapor pressure, it has also been found uponexamination to cause a higher chopping current level than is desirablein many cases. This unpreferred result is based on a good heatconductivity of the entire contact material, and though the minorconstituent, Bi, in the contact material, theoretically functions toreduce the heat conductivity, whenever the arc region of the contact isunable to achieve a higher temperature state because of a low loadcurrent, as, for example, 10 amperes or less, such a contact materialhas always shown the higher chopping current levels which are usuallyassociated with higher level surge voltages that cause damage to theinsulationsin the electrical installations connected to the vacuumswitches.

Besides, there are other significant problems in such vacuum typeswitches heretofore available and designed for reducing the choppingcurrent level by employment of electrode contact surfaces having highmetal vapor pressures.

In the first place, it will be readily appreciated that the arc regionof the electrodes may be quickly consumed due to the extremely hightemperature arcs which are initiated between the contact surfaces of theelectrodes during each current interruption, and which accompany theevolvment, or giving off, of the positive evaporations of the metalvapors. As is well known, these vacuum type switches are usuallyemployed so as to cause switching on and ofl of their accompanyingcircuits, for example, in an electric motor energization circuit, withhigh frequency of use. To this end, the electrodes are likely to beconsumed in an extremely short period of time, thereby shortening theeffective life of the vacuum switch.

Also, the higher temperature of the electric arc is likely to cause theelectrode to melt, and then spatter down into the envelope of the vacuumswitch. As a result, the consumption of the electrode will occur evenmore rapidly than might be expected from the aforesaid evaporation,whereby additional shortening of the life of the electric vacuum switchoccurs.

This spattering and melting of the electrodes may make many concaves onthe contact surfaces of the electrodes, and thus there may be made somesharp edges at the peripheral portions of the concaves, whereby thedielectric strength of the vacuum between the separated electrodes willbe reduced. In accordance with our experiences, the dielectric strengthof the vacuum type switch is reduced from the initial value to percentof that value after being subjected to several current interruptiontests. This means that such switches must be produced which are capableof withstanding voltages higher by more than 40 percent of the voltageratings thereof, and this involves an expensive manufacturing process.Finally, because the SUMMARY or THE INVENTION It is, therefore, anobject of the present invention to provide a vaccuum type switch havingseparable electrodes within a highly evacuated envelope in which atleast one of the electrodes includes an improved contact material at itscontact surface, which material is not characteristic of higher metalvapor pressures than conventional materials heretofore employed, yetwhich maintains an electric arc between separated electrodes when theelectrodes are separated to open an electric circuit connected to theswitch gear.

Another object of the present invention is to provide an electric vacuumswitch having separable electrodes within a highly evacuated envelopewhich possesses improved arc maintenance characteristics in low currentinterruptions.

In accordance with a preferred embodiment of the present invention, theforegoing and other objects are achieved by providing a pair ofseparable electrodes positioned within a highly evacuated envelope so asto be capable of making and breaking an electric circuit in which theyare connected. A contact surface on at least one the the electrodes ispreferably made of a sintered alloy including a mixture of a body ofmetal carbide having a low work function and at least one kind ofwettable material being composed of nickel, copper and cobalt inpreselected ranges as will be set forth in the following description.The sintered 1 alloy is impregnated with at least one type ofelectrically higher conductive metal by an amount of sufficientproportion capable of improving the electric conductivity and machiningcharacteristics of the entire contact material including said body ofmetal carbide.

BRIEF DESCRIPTION OF THE DRAWING Other objects and many of the attendantadvantages and features of the present invention will be readilyappreciated as the same becomes better understood from the followingdetailed description when considered in connection with the accompanyingdrawing, therein the sole FIG. shows an elevational sectional view of anelectrical vacuum switch constructed in accordance with the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to thedrawing, there is shown an electrical vacuum switch generally indicatedby the reference numeral 10. As is well known, the vacuum type switchconventionally comprises a highly evacuated envelope 11 having acylindrical side wall 12, which may be made of any suitable insulatingmaterial, such as, for example, glass, ceramics or the like, and a pairof separable contacts 13 and 14 made of an improved material accordingto the present invention and located within the envelope. One of thecontacts 13 is stationary and is secured to an inner end of a stationaryelectrode 15, the end of which extends through a metallic end cover, orplate 16 located on an upper peripheral portion of the cylindrical sidewall 12. A vacuum-tight seal member 17 is provided between theperipheral portions of the end cover 16 and one of the open ends of theinsulating side wall 12. The portion of the end cover 16 through whichthe stationary electrode 15 extends is vacuum-tightly sealed ofi by anysuitable sealing means, such as a weld. The uppermost end of thestationary electrode 15 is connected with a conductor, which, in turn,is connected to an AC. power supply (not shown).

The other contact 14 is a movable one, which is securely mounted on aninner end of a movable electrode 18, the other end of which is connectedto an AC. conductor of an AC. load circuit located outside of theenvelope, as well as to a driver which acts to reciprocably move theelectrode 18 and the contact 14 between positions which are alternatelyclose to and separate, or further away, from the stationary contact 13.Similarly, as described above, there is provided a vacuum-tight sealmember 19 for vacuum-tightly sealing the envelope 11 between theperipheral portions at the other open end of the cylindrical side wall12, opposite the cover 16, and a separate cover, or plate, 20. The platehas an opening 21 through which an outer end of the movable electrode 18slidably extends out of the envelope 11. To vacuum-tightly seal off theopening 21 through which the movable electrode extends, a flexiblemetallic bellows 22 is a vacuum-tightly secured at one end thereof tothe edge portion of the opening 21, and the other end of the bellows 22is vacuumtightly connected to the movable electrode 18 through a sealingcap member 23.

The vacuum type switch constructed in accordance with the presentinvention is not intended to rely solely upon the utilization offunction due to the higher metal vapor pressure to maintain an electricare 25 occurring between the separated contacts 13 and 14 when thedevice is switched off for interrupting the current flowing in theswitch circuit. However, where the contacts are opened for switchingoff, or interrupting the current flow, there will occur certain metalvapors between the separated contacts which arise from the compositionof the contact material as explained hereinafter. These metal vaporswill, however, be diffused into the vacuum space around the arc andthereby eventually result in final extinction of the arc. At the sametime, vapor deposition on the surface of the bellows, which might causeinjury to the mechanical characteristics of the bellows, is prevented bythe cap type seal 23 which effectively prevents the depositions of thevapors on the surface of the bellows through its specific sealingarrangement.

On the other hand, a part of the aforesaid vapors might be deposited onthe inner surface of the side wall 12 which is made of insulatingmaterial. This may result in the lowering of the dielectric strengthalong the axial direction of the side wall. To avoid this defect, thereis arranged a cylindrical shield member 24 around the contact area andisolated from the inner surface of the side wall 12 and the contacts 13and 14, respectively. Thus, the shield 24 will catch the metal vaporparticles randomly directed toward the inside wall of the envelope 11,and effectively preventing the aforesaid lowering of the dielectricstrength of the envelope wall.

In the drawing, there is shown an open status for contacts 13 and 14,thereby to define a gap therebetween for isolating the load circuit fromthe source.

When closing of the contacts 13 and 14 is required, it is only necessaryto move the movable electrode 18 with the movable contact 14 upwardly bymeans of the driver, not shown, whereupon the bellows 22 will becomeelongate to make the contacts without any breakdown of the vacuum in theenvelope. On the contrary, where it is necessary to separate the contact14 from the contact 13, the driver will drive the electrode 18downwardly, thereby causing the separation of the contact 14 from thecontact 13. In the latter case, there will occur a shrinkage on thebellows and an electric arc 25 having an extremely higher energy will beinitiated, as well as be drawn out, between the separated contacts.

As pointed out, the arc includes substantial metallic particles whichare evaporated from the contact surfaces and possess good electricconductive characteristics for normally maintaining the are even at lowcurrent levels. However, these metallic particles or vapors will rapidlydiffuse into the vacuum atmosphere around the arc, causing it to beextinguished quickly whenever the current is fully interrupted, whichnormally occurs at or close to the first current zero in the appliedalternating current wave form.

According to the preferred embodiment of the present invention, thecontact material is made of a sintered alloy comprising silver (Ag) andtungsten carbide (WC).

In the manufacture of the contact material, initially, there is preparedfor the compositions of the material in order to obtain the endconstituents for the contacts 13 and 14, a mixture as follows:

Ni0.l to 5 weight by percent Cu0.l to 1 weight by percent Co0.l to 5weight by percent Ag-l0 to 60 weight by percent Free Carbonless than0.05 weight by percent WC-the remainder Tungsten carbide (WC) has thecharacteristic of a low work function, that is, it possesses higherthermionic electron emission characteristics, and besides, it is a kindof refractory material. As is well known, tungsten carbide (WC) is alsodifiicult to machine for making the contacts.

Ni, Cu, and Co are the wettable materials for the tungsten carbide (WC),and they operate to improve the wettable characteristics of the tungstencarbide (WC) so as to make capable easier impregnation of the Ag intothe WC in every nook and corner, or among the interstitial spaces,thereof.

First, as is usually done, the powdered WC, Ni, Cu and Co are mixed, andthen they are sintered to make a sintered alloy. Then, the resultantsintered alloy is located within an exhausted atmosphere, and eithersunk in a bath filled with thev molten silver or has the base portionthereof immersed in the molten silver.

Thus, the molten silver can be impregnated among the particles oftungsten carbide to fill therewith all of the interstitial spaced amongthem. The aforesaid wettable materials greatly contribute to thisimpregnation process. In accordance with our repeated tests, this goodstate of the-impregnation has been confirmed in the fact that theresultant contact material has not shown the emanation therefrom of theionizable gases which might have existed among the WC particles when thecontact material has suffered from the higher temperature of electricarcs.

The examinations, or tests, on the current chopping level have beencarried out more than 30 times per each pair of electrodes with 10amperes. The chopping current level is the current amplitude at whichthe chopping phenomena occurs and it varies with each test. Usually themaximum value out of many test datum, for example, more than 30, isadapted as the current chopping level of the electrodes. The electrodesunder these tests had the dimensions of 22 mm. diameter, 5 mm. thicknessand 25 mm. diameter, 5 mm. thickness.

As the result, a sintered alloy including 30 percent by weight of Ag,has given a chopping level between 1.1 A. and 2.0 A. On the other hand,another alloy including 40 percent by weight of Ag has shown a choppinglevel between 1.9 A and 3.2 A. Besides, a further WC alloy including 55percent by weight of Ag has shown levels between 2.4 A and 4.2 A.

It can be thus seen that the less Ag present by percentage of weight,the. smaller will be the chopping current levels of the WC alloy.

Accordingly, it is desirable to reduce the amount of silver to beimpregnated within the sintered tungsten carbide (WC) alloy in order toreduce the chopping current level. However, we have found that thecontact material becomes harder to machine as the amount of silverimpregnated in the sintered WC alloy is decreased. In accordance withour experiences, when the contents are less than 10 percent by weight ofAg in the sintered WC, it is impossible to carry out the requiredmachining process of the contact material because of the occurrence ofbreaks in the cutting tool for the-material. On the other hand, when thecontents of Ag in the sintered WC are more than 60 percent by weight,another defect arises in that the contact material exhibits a higherchopping current level. This results from thereason that it is difficultto maintain the electric arc between the separated contacts because ofthe lower temperature involved due to smaller current conditions. Such alower temperature rise also results from the higher heat conductivity ofthe silver.

The silver impregnated within the sintered WC in accordance with thepresent invention functions to im prove the electric conductivity of thesintered alloy.

When the electrodes 15 and 18 having the contacts 13 and 14 areseparated in order to interrupt the electric circuit, an electric are 25is initiated between the separated contacts, and then it makestemperatures of the contact surfaces rise to cause the cathode spots onthe contact surfaces from which may be emanated the thermionicelectrons. AS mentioned above, the WC has a low work function, so thateven though the currents flowing through the are initiated between theseparated contacts have smaller amplitudes, there will neverthelessoccur an emission of the thermionic electrons sufiicient to ionize thesmaller amounts of silver vapor which exist between the separatedcontacts.

To this end, there will be positively maintained an are having smallercurrent levels between the separated contacts until the current hasfallen to a minimum level which will not cause chopping of the currentto the electric circuit in which the vacuum switch is connected. In thisway, perceptible surge voltages do not occur in the circuit to destroythe insulations of the electric installations connected thereto.

Likewise, in accordance with the present invention,

no concaves are created on the contact surface during the existence ofthe electric are because the contacts are mainly made of a refractorymaterial, WC. According any lowering of the dielectric strength betweenthe separated contacts due to the concaves does not occur with thisinvention. Further, even though some amounts of silver impregnated inthe contact material are evaporated due to higher temperatures sufferedfrom the arc, the low work function characteristics of the contactmaterial does not vary, and to this end the rising of the choppingcurrent levels does not occur, such as might have occurred in the priorvacuum type switch contact.

It is to be understood that modifications may obviously be made bysubstituting other types of contact materials, for example, Cu and WC.The contact made of the sintered alloy including Cu and WC (40 percentby weight being Cu) has shown a chopping current level of 4.2 A., whichis higher than those of the sintered alloy including Ag and WC. This isbecause the low work function of the Ag is extended over a range between3.08 to 3.56 e.g., while on the other hand the low work function of theCu is extended over the other range between 3.85 to 4.38 e.v., and thevalues of the latter are higher than those of the former. Further, the

vapor pressure of the Ag is higher than that of the Cu, comparing at thesame temperature.

Also, it should be readily apparent that the low work function materialWC may be replaced by MoC, ZrC, TiC, VC and SiC, as well as thecombinations of these. It should be further noted that the amount of thefree carbon involved in the contact material is preferably less than0.05 percent by weight for the purpose of preventing the lowering of thewettable characteristics of the contact material.

Obviously, many modifications and variations of the present inventionare possible in light of the teachings herein. It is understood,therefore, that within the scope of the appended claims, the inventionmay be practiced otherwise than as specifically described herein.

What is claimed and desired to be secured by Letters Patent is:

1. In a vacuum type switch for maintaining an arc at low current levelscomprising a highly evacuated envelope and a pair of separable contactslocated in the envelope so as to make and break an electric circuit, theimprovement comprising:

at least one of the separable contacts being formed of a sintered metalcarbide being selected from a group consisting of WC, MoC, ZrC, TiC, VC,SiC, and the combinations thereof, and including a wettable materialcomposed of 0.1 to 5 percent by weight of Ni,

0. l to 1 percent by weight of Cu and with an electrically conductivemetal amounting to between 10 and 60 percent by weight of the totalweight of said sintered metal carbide, said wettable material and saidelectrically conductive material. 2. In a vacuum type switch accordingto claim 1, said electrically conductive material being silver (Ag).

1. In a vacuum type switch for maintaining an arc at low current levelscomprising a highly evacuated envelope and a pair of separable contactslocated in the envelope so as to make and break an electric circuit, theimprovement comprising: at least one of the separable contacts beingformed of a sintered metal carbide being selected from a groupconsisting of WC, MoC, ZrC, TiC, VC, SiC, and the combinations thereof,and including a wettable material composed of 0.1 to 5 percent by weightof Ni, 0.1 to 1 percent by weight of Cu and 0.1 to 5 percent by weightof Co, the sintered metal carbide further being impregnated with anelectrically conductive metal amounting to between 10 and 60 percent byweight of the total weight of said sintered metal carbide, said wettablematerial and said electrically conductive material.
 2. In a vacuum typeswitch according to claim 1, said electrically conductive material beingsilver (Ag).